Zirconia nanoparticles have a high refractive index and are useful in organic matrices to alter optical properties of the matrix. For example, zirconia nanoparticles have been used to increase the index of refraction or to increase the x-ray opacity of the organic matrix, while retaining optical transmission. The extent to which the x-ray opacity and/or refractive index of the organic matrix can be increased is dependent on the percent loading of zirconia in the organic matrix and on characteristics of the zirconia particles such as the percent crystallinity, the crystalline structure, the primary particle size, and the degree of association between the primary particles.
Surface modification of zirconia nanoparticles can be used to prevent or reduce particle agglomeration and to enhance the compatibility of the nanoparticles within an organic matrix. Accordingly, zirconia nanoparticles have been treated with a variety of surface modifying agents such as, for example, carboxylic acids and/or silanes. These traditional surface modifiers have their drawbacks. For example, organic matrices containing acrylic acid-derived residues will displace the zirconia-bound carboxylic acid groups with acrylic acid-derived groups. Silane-functionalized zirconia nanoparticles are thermodynamically unfavorable and experimentally challenging to prepare.